This application claims benefit of Japanese Patent Application No. 2003-272858 filed on Jul. 10, 2003, the contents of which are incorporated by the reference.
The present invention relates to portable image receiving devices and portable terminal devices such as portable image receiving devices capable of extending the life time of a power supply battery.
In our current daily life, telephones and televisions (TV) receivers have become essential electronic devices. Various portable models of these devices have been developed with advancement of semiconductor techniques and realization of both color and improved performance liquid crystal displays (LCD). Portable image receiving devices capable of television reception have been reduced in size and weight such that they are now capable of being put on a palm-sized device with realization of image display on a display unit (such as LCD) of several inches.
The portable telephone, PHS (personal handy-phone system) telephones and PDA (personal digital assistant) devices also have reduced in size and weight and have extended usage times. Particularly, mobile telephones and PHS telephones have been improved with size and weight reduction down to 100 grams and standby battery life extensions of about 500 hours. These improvements have been outstandingly popular.
However, as of now, most of the above portable devices are not integrated into one unit. And, even devices adapted to provide multiple functions in the same unit mostly incorporate several, subordinate functions that are intrinsic main functions. For example, a mobile phone may have an electronic telephone diary function, a mail function, a memo function, a game function, etc. that are provided together with the telephone functions.
Recently, high density fitting has become possible with semiconductor devices, and mobile phones and PHS telephone sets with additional digital camera functions are commercially available and popular. Thus, it is probable that composite devices having a plurality of main functions will increase in number in the future. Particularly, it is probable that with realization of their size and weight reduction, devices having both telephone functions and TV receiving functions become popular products in the future.
However, TV functionality in a mobile device requires high consumption current and the broadcast is often received for long time. Therefore, when a battery is used as power source for a portable television, the broadcast can be received for at most only about two hours from the fully charged state of the battery. Due to this limited length of possible broadcast reception time, the integration of television functions within mobile, phones and the like has not been commercially viable. For example, in the case of a mobile phone with TV functions, the telephone functions would be useless after a 2 hour television broadcast. Thus, it is necessary to reduce the consumed current in these devices as much as possible during television operation.
FIGS. 7(a) to 7(e) show the operation of one type of prior art portable image receiving device. When the power supply is turned on, as shown in FIG. 7(a), by operation of a power supply switch in the portable image receiving device, a TV receiver is furnished with power as shown in FIG. 7(c), and reception of TV waves is thus begun. When the portable receiving device is used outdoors, the radio wave receiving status is influenced in dependence on such factors as the place of reception, and the reception level is subject to variations. Particularly, the reception level is subject to variations in reception while the unit is moving. When the radio wave receiving status is good and the reception level is high as in a time section T1 in FIG. 7(b), both the TV video and TV audio can be reproduced without noise, and no problem arises in the TV reception. However, when the portable receiving device is moved, the radio wave receiving status is deteriorated as in time section T2 in FIG. 7(b). When the TV reception level becomes lower than a certain level, as shown in FIGS. 7d and 7e, noise becomes noticeable in both the TV video and audio. In this status, the TV broadcast reception is no longer good.
However, the circuitry of the prior art portable receiving device is constructed such that it reproduces both the TV video and audio at all times irrespective of the radio wave receiving status so long as the power supply is “on”. In other words, even the radio wave receiving status is so bad that TV broadcast reception is no longer good, the power is still supplied to all the circuits in the device. Thus, in the device using a battery as power supply, the battery is drained even when broadcast reception is poor. Several prior art devices attempt to remedy this problem.
FIGS. 8(a) to 8(f) show the operation of a prior art portable receiving device with the control of the power supply based on the radio wave receiving status. In this operation, after the power supply has been turned on as shown in FIG. 8(a), the electric field intensity of audio signal during the TV reception is observed (i.e., detected), and when the detected electric field intensity becomes lower than a predetermined threshold level, the power supply to the TV audio and video receiving circuits is turned off as shown in FIG. 8(c). Thus, the video and audio outputs are brought to zero as shown in FIGS. 8(d) and 8(e). When the detected electric field intensity exceeds the threshold level, the power supply to the TV audio receiving circuit and a TV video receiving circuit and is turned on, thus reducing the consumed current in the device (see Literature 1: Japanese Utility Model Laid-open Heisei 01-103977).
Another TV receiver has been proposed, in which the end of broadcast is decided by checking whether or not received waves are present, and after the lapse of a predetermined period of time from the cessation of the received waves is detected, the power supply is entirely is turned off (see Literature 2: Japanese Utility Model Laid-open Heisei 01-132184).
However, these prior art television receivers, require observation of the reception level at all times, including when the power supply to the TV audio and video receiving circuits are “off”. This means that the circuit for receiving the TV signal wastefully drains current from the power supply, e.g., the battery.
Additionally, in the prior art devices, stable TV signal reception may not be obtained when the device is moving due to increased noise which detracts from the user's enjoyment of the TV reception. Although TV reception can be enjoyed when radio waves are received, current is consumed by the receiving system circuit despite the lack of reception, and the same can not be enjoyed until the radio wave reception is good. Consequently, batteries for these devices wear-out quickly.
And, while the TV function may be turned off by the user when noise is increased, the TV function must be turned back on to see whether the radio wave receiving status is good. Thus, portable TV devices that reduce current consumption as much as possible while also taking the user's convenience into consideration are needed.